Calcium can function as a signal which activates gene expression via stimulation of calcium-dependent protein kinases. Calcium-dependent protein kinases (CDPKs) represent a family of protein kinases which are proposed to contain, in a single polypeptide, both a kinase domain and an adjoining calmodulin-like domain with four calcium-binding motifs (Harper, J. F., et al. (1991) Science 252:951-954). Some CDPK proteins kinase have been isolated that require calcium but not calmodulin for activity. Research has shown that multiple CDPK isoforms are present in Arabidopsis thaliana and other plants and that plant CDPKs may play a pivotal role in the regulation of many cellular process such as stress response (WO 98/26045) and male gametophyte formation (WO 97/35968).
Glycogen synthase kinase-3 (GSK-3) in animal cells is a serine/threonine kinase protein, which is involved in the regulating the activity of several transcription factors including the DNA-binding activity of the c-jun/AP1 transcription factor. AP1 is a transcription factor that recognizes a specific enhancer target DNA sequence and when bound to enhancer regions stimulates promoter transcriptional activity. AP1 is composed of several polypeptides of which c-jun is the major component. Several plant GSK-3 proteins have been identified that have homology to GSK-3 gene family of protein kinases (Bianchi et al. (1994) Mol. Gen. Genet. 242(3):337-345).
There is a great deal of interest in identifying the genes that encode protein kinase enzymes involved in the control of gene expression in plants. These genes may be used to modulate or control protein expression in plant cells. Accordingly, the availability of nucleic acid sequences encoding all or a portion of a regulatory protein kinase would facilitate studies to better understand gene regulation in plants, and provide genetic tools to permit more accurate control and manipulation of gene expression in plants.